This invention relates to the pressure bonding of two components together with a curable adhesive and malleable metal spheres, and more particularly to such bonding where one or both of the components are fragile.
In one form of an infrared sensor device, a sensor chip assembly (SCA) is bonded to the mounting platform of a cryostat to form a bonded assembly. The sensor chip assembly is the active sensor component, and the cryostat is supported on an aiming structure such as a gimbal. The detector plane of the sensor chip assembly must be aligned extremely precisely with respect to a reference datum plane, to ensure that the sensor chip assembly is pointed in the proper direction by the aiming structure during service. In some designs, alignments must be achieved between the two planes on the order of 10 arcseconds or better.
The sensor chip assembly is a microelectronic device which includes a detector and a readout that are fabricated by microelectronic techniques, and then joined together. The sensor chip assembly is a relatively fragile structure that may be easily damaged by even relatively small forces that exceed its load limit. Such damage may include mechanical cracking or breaking, or the damage may be more subtle in the form of a degradation in electrical performance without cracking or breaking. The mounting platform to which the sensor chip assembly is bonded usually is a more-robust mechanical structure.
To accomplish the bonding, the sensor chip assembly and the mounting platform are carefully aligned in a bonding apparatus. The bonding apparatus is an ultra-high-precision device which is custom adjusted for each bonded assembly to account for small structural variations that would otherwise result in an unacceptable misalignment of the sensor chip assembly and the reference datum plane in the absence of care taken to ensure proper alignment. Current practice involves dispensing a quantity of curable adhesive between the sensor chip assembly and the mounting platform, and the two components are pressed together and constrained in place by the bonding apparatus until the adhesive is cured.
The bonding apparatus is used to maintain the correct alignment until the adhesive cures sufficiently past the stage at which it may creep and deform during curing, which may require several hours. The alignment must be forcefully maintained because, if the bonded structure were removed from the bonding apparatus too early and before curing is complete, the slight shape change of the adhesive during curing could produce a misalignment between the sensor chip assembly and the reference datum plane that would be unacceptable. The bonding apparatus provides sufficient constraint during curing of the adhesive and prevents such shape change and misalignment.
While this approach is operable and works well in a laboratory setting, it is uneconomical in a production operation. In production, the precision bonding apparatus is tied up with each individual assembly for an excessively long time by the need to prevent movement of the sensor chip assembly until the adhesive cures sufficiently, which may be several hours. The bonding apparatus is quite expensive, and it is therefore not possible to supply a large number of them consistent with the production requirement to fabricate a large number of the bonded assemblies. There is therefore a need for an approach that produces the required alignment of the bonded components in the assembly, but which is more suitable for a production operation. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for fabricating a structure from two (or more) components. It is particularly useful when one or more of the components is relatively fragile, as in the case where one of the components is a microelectronic device such as a sensor chip assembly. Damage to such fragile components is avoided by carefully controlling the loads applied to the structure during the bonding operation. The present approach is highly reproducible and does not require the judgments of a skilled technician during the bonding operation.
In accordance with the invention, a fabrication method comprises the steps of providing a first component and a second component, positioning the first component in facing-but-spaced apart relation to the second component, and placing a bonding medium between the first component and the second component. The bonding medium comprises at least two malleable spheres made of a metal that bonds to both the first component and to the second component when subjected to a sufficiently large force, and a quantity of an uncured adhesive. The first and second components may be of any operable types. In a case of interest, the first component is a sensor chip assembly. The second component may be a mounting platform. A preferred metal for use in the spheres is malleable indium metal.
The method further includes bonding the first component to the second component using the bonding medium by supplying a bonding apparatus having at least one force actuator. The bonding apparatus presses the first component against the second component in a facing-but-spaced-apart relation, with the bonding medium therebetween and with a sufficient bonding force to bond the malleable spheres both to the first component and to the second component. There may be at least two independently controllable force actuators, or a single force actuator may be used. Simultaneously with the pressing, at least one measured bonding reaction of the first component and the second component is monitored, and the force actuators of the bonding apparatus are controlled responsive to the step of monitoring. The adhesive is thereafter cured.
In one particularly preferred embodiment, the steps of placing and the bonding apparatus pressing are performed cooperatively by positioning the first component and the second component in a facing relationship in the bonding apparatus, dispensing the uncured adhesive between the first component and the second component, positioning the spheres in the adhesive, bringing the first component, the second component, and the spheres into touching contact with each other, and forcing the first component toward the second component with sufficient force to bond the spheres to the first component and to the second component.
In the basic process, the step of controlling preferably includes the steps of providing a set of bonding reaction limitations, comparing the measured bonding reactions with the respective set of bonding reaction limitations, and sending control signals to the at least one force actuator responsive to the step of comparing. The step of providing a set of bonding reaction limitations preferably includes evaluating a set of stresses that cause damage to the first component, and selecting the set of bonding reaction limitations responsive to the step of evaluating the set of stresses. The bonding apparatus loading may otherwise be conducted according to a preselected load profile. The step of controlling may also include the step of determining a set of maximum stresses applied to the first component.
In the preferred processing, the step of curing the adhesive includes the step of removing the bonding force prior to completion of full curing of the adhesive. That is, the bonding spheres are placed in the adhesive, and the bonding apparatus is used to force the components together into their final aligned positions. The bonded structure is thereafter removed from the bonding apparatus and set aside to allow the adhesive to cure. The components are held together in the desired position and orientation by the adhesion of the malleable spheres to the components until the adhesive fully cures, avoiding the need to keep the components in the bonding apparatus until the adhesive is cured. The spheres rather than the bonding apparatus provide the necessary constraint during the remainder of the curing process. Process throughput is greatly increased, reducing the manufacturing costs for the product.
The use of malleable metals and adhesives between electronic components is known in other contexts, such as in the bonding of an infrared detector to a readout integrated circuit in a hybridization or xe2x80x9cflip chipxe2x80x9d using deformable metallic bumps. However, the malleable metals are not employed in a bonding operation like that of the present approach, where they constrain the deformation of the adhesive during curing. That is, in these other approaches an uncured adhesive is not applied prior to the bonding operation, and the bumps do not serve to constrain the adhesive during the curing of the adhesive.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.